Abstract: In order to understand the mechanism of expansive soil canal slope slide, a study is conducted based on the statistical results of 19 landslides at the Nanyang Section of Middle Route of South-to-North Water Transfer Project. The right bank landslide at TS105+400 is selected as typical landslide. Excavating trench exposes internal structure of landslide. A series of geological survey has been done to study the characteristics and evolution mechanism of expansive soil landslide in the area by excavating trench slides. The results show that, most landslides in the area occurred in the strata. Slope stability is jointly controlled by vertical joints in the upper layer soil and the gently dipping big fissures at the toe of the slope, which are filled with highly expansive clay. Sliding surface is composed of the steep tilting cracks at the trailing edge of slope and big gently dipping fractures at the toe of slope. Excavation unloading effect causes the vertical joints’ opening. These joints can extend more than 3 meters into the slope, which destroys the integrity of the slope soil and serves as the main channel for water to go in and out of the slope; there is a high humidity area 4 to 8 meters below the surface of slope. Soil within the area is weak and the sliding surface develops in it. Wetting and drying cycles caused by climate and excavation unloading lead to the vertical crack to open and propagation deep into slope. This effect causes a dramatic attenuation of soil strength. The vertical cracks connected with the big gently dipping fissures at the toe of the slope eventually. The slope slides after these fissure and crack are filled with water after the occurrence of strong lasting precipitation.

Key words: expansive soil, canal slope, landslide, genetic mechanism